1. Technical Field
The present disclosure relates to an integrated microelectromechanical gyroscope, in particular of a triaxial type, with an improved driving structure.
2. Description of the Related Art
As is known, current micromachining techniques enable production of microelectromechanical systems (MEMS) starting from layers of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed via chemical etching. Inertial sensors, accelerometers and gyroscopes obtained with this technology are encountering an increasing success, for example, in the automotive field, in inertial navigation, or in the field of portable devices.
In particular, integrated gyroscopes made of semiconductor material obtained with MEMS technology are known. These gyroscopes operate based on the theorem of relative accelerations, exploiting the Coriolis acceleration. When a rotation at a certain angular velocity (the value of which is to be detected) is applied to a mobile mass that is driven with a linear velocity, the mobile mass “feels” an apparent force, called the “Coriolis force”, which determines a displacement thereof in a direction perpendicular to the direction of the linear driving velocity and to the axis about which the rotation occurs. The mobile mass is supported via elastic elements that enable a displacement thereof in the direction of the apparent force. According to Hooke's law, the displacement is proportional to the apparent force, in such a way that, from the displacement of the mobile mass, it is possible to detect the Coriolis force and the value of the angular velocity of the rotation that has generated it. The displacement of the mobile mass can, for example, be detected in a capacitive way, determining, in a condition of resonance, the capacitance variations caused by the movement of mobile sensing electrodes, which are fixed with respect to the mobile mass and are coupled (for example, in the so-called “parallel-plate” configuration, or else in a combfingered configuration) to fixed sensing electrodes.
MEMS gyroscopes generally have a symmetrical sensing structure, comprising a pair of sensing masses for each sensing axis about which it is possible to detect a rotation at a corresponding angular velocity. Ideally, an altogether symmetrical structure enables complete rejection, by using appropriate differential reading schemes, of externally applied linear accelerations of disturbance, for example due to shocks acting on the sensor or to the acceleration of gravity. In fact, whereas the Coriolis force tends to unbalance in opposite directions and substantially by the same amount the sensing masses of each pair (generating movements “in phase opposition”), external interferential accelerations of disturbance determine displacements thereof in the same direction and once again by the same amount (generating movements “in phase”); by subtracting the electrical signals associated to the two sensing masses of each pair it is ideally possible to measure the contribution due to the Coriolis force and to completely reject the contributions of the accelerations of disturbance.
MEMS gyroscopes moreover have a driving structure, which is mechanically coupled to the sensing structure, in such a way as to impart upon the sensing masses a linear driving velocity along a corresponding driving direction; the driving structure is set in motion by means of driving electrodes, which receive appropriate electrical biasing signals from an electronic driving circuit of the MEMS gyroscope. In particular, the biasing signals are such as to cause, by means of mutual and alternating attraction of the driving electrodes associated to a given driving mass, self-oscillation of the same driving mass in the corresponding driving direction, at a given frequency of oscillation (corresponding to the frequency of mechanical resonance of the driving mass).
The electronic driving circuit of the MEMS gyroscope comprises, in a known way, a complex feedback control loop for imparting the motion on the driving masses and controlling the resultant driving motion thereof (in particular, the frequency and amplitude of the corresponding oscillation), by means of feedback sensing structures (in particular, driving-sensing electrodes, which are designed to measure, through a variation of transduced charge, the displacement of the driving mass in the driving direction).
In general, and as described, for example, in U.S. Pat. No. 8,375,789, which is incorporated by reference herein, for each driving direction, the electronic driving circuit comprises a respective feedback control loop, constituted, amongst other elements, by charge-amplifier blocks, phase retarders, filters, oscillators, variable-gain amplifiers, PLL (phase-locked loop) stages, and, as a whole, is able to control driving of the driving masses in the corresponding driving direction.
It is hence evident that design and implementation of a microelectromechanical gyroscope are particularly complex, both as regards the micromechanical part and as regards the associated driving and reading electronics. In particular, as regards driving of a triaxial gyroscope, the requirement of envisaging a number of control loops (one for each driving direction) entails in general a considerable amount of resources and an associated high occupation of area in an integrated implementation. In addition, it is generally complex to maintain the desired ratios between frequency, phase, and amplitude of the oscillations of the driving masses in the various driving directions.